The frequency control system used in thermal power plant can produce good economic benefit. Based on the purpose of keep hydrodynamic coupling still and variable frequency speed regulation, this paper put forward pump wheel speed regulation method for hydrodynamic coupling. The result shows that the frequency control is the best way to realize this method for hydrodynamic coupling. And frequency control for motor-driven feed water pumps can be effectively implemented in 200, 300 and 600 MW unit with the utilization of this method, which can save more than 20% of electricity.